1. Field of the Invention
The present invention relates to a method of wavelength selection control and a wavelength variable filter device employing the method, particularly, to a method of wavelength selection control for an acousto-optic tunable filter, and a wavelength variable filter device employing the method.
2. Description of the Related Art
Optical communication devices capable of long-distance and large capacity optical communications are required in order to construct future multimedia networks. Among methods for realizing large capacity communications, a Wavelength Division Multiplexing (WDM) transmission scheme is being studied extensively because the WDM scheme is able to efficiently utilize the wide bandwidth and large capacity of optical fibers.
In an optical communication network, it is necessary to provide, at appropriate places in the network, functions of transmitting, dropping, and adding optical signals, and functions of optical routing and optical cross-connect for selecting optical transmission paths. For this purpose, research and development are being made of Optical Add Drop Multiplexer (abbreviated as “OADM”, hereinafter) devices for transmitting, dropping, or adding optical signals. The OADM devices include wavelength-fixed OADM devices capable of dropping or adding optical signals having fixed wavelengths and wavelength variable OADM devices capable of dropping or adding optical signals having any wavelength.
Meanwhile, because an acousto-optic tunable filter (abbreviated as “AOTF”, hereinafter) is operated to extract a light beam having a wavelength to be selected, the AOTF is able to select wavelengths arbitrarily, while a fiber grating has a fixed wavelength to be selected. In addition, because the AOTF also operates as a filter able to select variable wavelengths, it can be used as a variable wavelength selecting filter in a tributary station for adding or dropping optical signals in a terminal. For this reason, research and development are being made of an OADM device using the AOTF.
In the optical communication network, it is expected that a new optical burst switching transmission scheme will replace an optical stream transmission scheme in the related art. The optical burst switching scheme is proposed because the Internet is a burst network that transmits burst data, and thus the Internet traffic has high statistics. Therefore, by assigning wavelengths only in time periods necessary to burst data transmission and at intervals shorter than milliseconds, an optical transmission network having high network resource utilization can be constructed.
The optical burst switching transmission scheme is able to improve utilization of the network resources. The optical burst switching transmission scheme requires wavelength switching to be executed at an order of milliseconds or less. Because the AOTF is capable of wavelength switching on the order of micro-seconds or less, as it is known, the AOTF can be used as a core device in the optical burst switching transmission scheme.
FIG. 1 is a diagram illustrating a principle of operations of the AOTF.
In FIG. 1, two light guides 1-1, 1-2 are formed by titanium diffusion in a substrate 1-7 made of Lithium Niobate (LiNbO3), which is a ferroelectric crystal and exhibits a piezoelectric effect. The two light guides 1-1, 1-2 intersect at two places, and two light guide-type polarization beam splitters (PBS) 1-3, 1-4 are arranged at the two intersecting portions, respectively.
Between the two intersecting portions, a SAW (Surface Acoustic Wave) guide 1-6 made from a metal is arranged on the two light guides 1-1, 1-2. The SAW guide 1-6 propagates a surface acoustic wave, which is generated when a high frequency signal (160 MHz to 190 MHz) (referred to as a “RF signal” hereinafter) from a RF signal generator 1-10 is applied to an inter-digital transducer (abbreviated to be “IDT” hereinafter).
As illustrated in FIG. 1, light beams which have wavelengths λ1, λ2, and λ3, respectively, and each of which includes mixed polarization modes of a TE mode and a TM mode, are input to a port 1 of the AOTF. The PBS 1-3 splits each of the input light beams into a TE mode light beam and a TM mode light beam, and transmits these light beams to the light guides 1-1, 1-2, respectively.
When a RF signal f1 having a specified frequency is applied, the surface acoustic wave propagates through the SAW guide 1-6. At intersecting portions between the SAW guide 1-6 and the light guides 1-1, 1-2, because of an acousto-optic (AO) effect, the refractive indexes of the light guides 1-1, 1-2 vary periodically.
For this reason, out of the three input light beams, the light beams having specific wavelengths enabling interaction with the periodical variation of the refractive indexes experience rotation of the polarization modes thereof, and due to the rotation, the TE mode and the TM mode are exchanged. The angle through which the polarization mode rotates is proportional to power of the RF signal and an interaction length of the interaction of the TE mode light beam and the TM mode light beam with the periodical variation of the refractive indexes.
The interaction length is adjusted in correspondence to an interval between an absorber 1-8 and an absorber 1-9, which sandwich the IDT 1-5 and absorb the surface acoustic wave appearing on the light guides 1-1, 1-2.
Therefore, by optimizing the power of the RF signal and the interaction length, the TM mode light beam having the specific wavelength is converted to the TE mode light beam in the light guide 1-2, and the TE mode light beam having the specific wavelength is converted to the TM mode light beam in the light guide 1-1. The traveling directions of the thus obtained TE mode light beam and the TM mode light beam are changed by the PBS 1-4. Consequently, the light beam which has the specific wavelength enabling the interaction is selected as a separated light beam and is output from a port 3 of the AOTF. Meanwhile, the light beam which has a wavelength not enabling the interaction just transmits and is output from a port 2 of the AOTF.
In FIG. 1, it is exemplified that due to the RF signal f1, the light beam having the wavelength of λ1 is subjected to the interaction and is selected as a separated light beam.
As described above with reference to FIG. 1, the AOTF is able to select and separate a light beam having the wavelength (λ1) in correspondence to the frequency of the RF signal, and with the frequency of the RF signal being changed, the AOTF can change the wavelength of the light beam to be selected.
The output light beam from the port 2 corresponds to the light beams input to the port 1 with the light beam (λ1) having the wavelength in correspondence to the frequency of the RF signal being removed, that is, the light beam output from the port 2 corresponds to the light beams (λ2) and (λ3). In other words, the AOTF has a rejection capability.
FIG. 2 is a block diagram illustrating an example of a wavelength variable filter in the related art.
FIG. 3 is a block diagram illustrating an example of the register unit 20 of the wavelength variable filter in FIG. 2.
FIG. 4 is a flowchart illustrating operations of wavelength selection in the related art.
Here, it is assumed that WDM transmission signals under consideration have wavelengths of λ1, λ2, . . . , λn−1, λn, respectively, and the wavelengths λ1, λ2, . . . , λn−1, λn are distributed consecutively and at regular intervals.
As illustrated in FIG. 2, a wavelength variable filter 10 includes a 5-channel drop-type integrated AOTF 12, optical taps (optical splitters) 14a through 14e, an optical monitor circuit 16, a controller 18 formed from a digital signal processor (DSP), a register unit 20 formed from a field programmable gate array (FPGA), and a RF signal generation circuit 22.
The WDM transmission signals having wavelengths of λ1, λ2, . . . , λn−1, λn are split by a splitting coupler 24, and are input to ports P1 through P4. A signal having a reference wavelength of λref1 at the minimum wavelength side and a signal having a reference wavelength of λref2 at the maximum wavelength side are input to a port P5 of the AOTF 12. For example, the reference wavelength λref1 may be set to be shorter than the minimum wavelength (λ1) of the WDM transmission signals by a value equivalent to one channel, and the reference wavelength λref2 may be set to be longer than the maximum wavelength (λn) of the WDM transmission signals by a value equivalent to one channel. This is illustrated in step S10 in FIG. 4.
The RF signal generation circuit 22 decreases the frequency of the RF signal supplied to the AOTF at the port P5 from 180 MHz, for example, each time by 1 kHz. The light beam output from the port P5 is split in the optical splitter 14e, and is converted to an electrical signal in the optical monitor circuit 16. The voltage values of the obtained electrical signal are input to the controller 18 as detected values of the reference wavelength λref1, which is the reference wavelength at the minimum wavelength side, and the reference RF frequency f1 when the controller 18 detects a maximum value is set to be in correspondence to the reference wavelength λref1. The obtained reference RF frequency f1 is recorded in a register 20a of the register unit 20, as shown in FIG. 3. This is illustrated in step S11 in FIG. 4.
On the other hand, the RF signal generation circuit 22 increases the frequency of the RF signal supplied to the AOTF at the port 5 from 160 MHz each time by 1 kHz. The light beam output from the port P5 is split in the optical splitter 14e, and is converted to an electrical signal in the optical monitor circuit 16. The voltage values of the obtained electrical signal are input to the controller 18 as detected values of the wavelength λref2, that is, the reference wavelength at the maximum wavelength side, and the reference RF frequency f2 when the controller 18 detects a maximum value is set to be in correspondence to the reference wavelength λref2. The obtained reference wavelength λref2 is recorded in the register 20a of the register unit 20. This is illustrated in step S12 in FIG. 4.
The controller 18 subtracts the reference RF frequency f2 from the reference RF frequency f1 to calculate a RF frequency interval. This is illustrated in step S13 in FIG. 4.
Further, the controller 18 calculates the number of channel intervals (n+1=n+2−1) from the number of channels of the WDM transmission signals (it is n) and the number of the reference wavelengths (it is 2). This is illustrated in step S14 in FIG. 4.
Next, the controller 18 sets the RF signal generation circuit 22 so that a RF signal having the reference RF frequency f1, which corresponds to the reference wavelength λref1 at the minimum wavelength side, is generated and supplied to the ATOF at the port 5.
Without being influenced by variation of the environment temperature or fluctuation of the intensity of the light source having the reference wavelength, the controller 18 performs frequency-tracking and power-tracking to optimize the RF frequency such that the detected value of the RF frequency, that is, the voltage value of the electrical signal generated in the optical monitor circuit 16 by opto-electrical conversion, becomes the maximum. With the optimized RF frequency, the controller 18 updates the reference RF frequency f1 and the corresponding RF power recorded in the register 20a. 
The controller 18 updates the reference RF frequency f2 corresponding to the reference wavelength λref2 at the maximum wavelength side by taking into consideration a variation of the reference RF frequency f2 relative to the reference RF frequency f1 corresponding to the reference wavelength λref1 at the minimum wavelength side.
In addition, the controller 18 subtracts the updated reference RF frequency f2 from the updated reference RF frequency f1 to calculate and update the RF frequency interval. This is illustrated in step S15 in FIG. 4.
When the controller 18 receives a request for wavelength selection from a device at an upper level, as illustrated in step S16 in FIG. 4, the controller 18 calculates a dependent RF frequency of the channel to be selected from the RF frequency interval and the number of channel intervals. For example, if the wavelength to be selected by the port P1 is λ2, the controller 18 divides the RF frequency interval by the number of the channel intervals, then the result multiplied by two is subtracted from the reference RF frequency f1 to calculate the RF frequency interval. In this way, the controller 18 obtains the dependent RF frequency of the selected channel λ2. This is illustrated in step S17 in FIG. 4.
The obtained dependent RF frequency and the RF power are recorded in the register 20b, which is used by the port P1, of the register unit 20 shown in FIG. 2 to set RF frequency to be generated by the RF signal generation circuit 22. In this way, the selected channel λ2 is selected at the port P1.
In order that the reference RF frequency be optimized without being influenced by the environment temperature variation, and the intensity fluctuation of the light source having the reference wavelength, the controller 18 performs frequency-tracking and power-tracking at the port P1 such that the voltage value of the electrical signal generated in the optical monitor circuit 16 becomes the maximum. Based on the variation of the optimum RF frequency, the controller 18 updates the reference wavelengths λref1 and λref2 at the minimum side and at the maximum wavelength side, respectively, by taking into consideration relative variations of the reference RF frequencies f1, f2, which correspond to the reference wavelengths λref1 at the minimum wavelength side and the reference wavelength λref2 at the maximum wavelength side, respectively. Further, based on the updated results, the RF frequency interval is also updated. This is illustrated in step S18 in FIG. 4.
For example, the related art of the present technical field is described in the following references.
Japanese Laid Open Patent Application No. 2000-241782 discloses a technique of calculating a wavelength control frequency corresponding to a specified wavelength from the upper and lower reference wavelengths to perform AOTF wavelength control.
Japanese Laid Open Patent Application No. 11-98122 discloses a technique of constantly updating wavelength correction control data to perform wavelength control of a wavelength variable filter.
Japanese Laid Open Patent Application No. 11-289296 discloses a technique of reading out wavelength control data stored beforehand to perform AOTF wavelength control.
Japanese Laid Open Patent Application No. 2003-344817 discloses a device including an AOTF, a RF signal generation circuit, a split light monitor circuit, and a signal processing circuit, wherein the RF signal generation circuit includes a direct digital synthesizer and a frequency multiplication processor.
Japanese Laid Open Patent Application No. 11-218790 discloses a device capable of adding, dropping, and transmitting optical signals having any frequency and any number of multiple codes.
In recent years and continuing, in the field of WDM transmission techniques, the optical burst switching transmission technique is being studied in order to achieve efficient optical stream data transmission. In optical burst switching transmission, it is required to perform high speed wavelength switching at a time period shorter than 50 μs. For this purpose, an AOTF device capable of fast operations is being studied.
However, in the related art as illustrated in FIG. 2 through FIG. 4, in order to calculate a correspondence relation between a wavelength and a RF frequency or RF power, each time wavelength selection is to be performed, the controller 18 needs to analyze a wavelength selection request, perform calculations, access the register unit 20, and perform frequency-tracking and power-tracking for the reference RF. Due to this, the operation of wavelength selection and switching costs about 2 ms, and it is difficult to realize high speed optical wavelength switching.